The present invention relates generally to ground proximity warning systems for use in aircraft. More particularly, the apparatus, methods, and computer program products of the present invention relate to determining a look ahead distance value for use in a ground proximity warning system that provides a desired reaction time for high speed flight.
An important advancement in aircraft flight safety has been the development of ground proximity warning systems. These warning systems analyze the flight parameters of the aircraft and the terrain surrounding the aircraft. Based on this analysis, these warning systems provide alerts to the flight crew concerning possible inadvertent collisions of the aircraft with surrounding terrain or other obstacles.
Although these warning systems are quite useful in providing the flight crew with information concerning potential problems with the navigation of the aircraft, the usefulness of these systems must be balanced against problems associated with the generation of nuisance alarms. Specifically, although it is advantageous to provide the flight crew with as much information as possible concerning terrain and obstacles in the flight path of the aircraft, this information should be delivered to the flight crew in a timely manner, such that the flight crew will pay close attention to these alarms. If alarms are provided too far in advance to the flight crew concerning terrain that is still far away from the present position of the aircraft, the flight crew may become desensitized to the alarms and may potentially ignore alarms from the ground proximity warning system altogether. Further, the generation of alarms concerning terrain that is still far away from the present position of the aircraft may also add stress and confusion on the flight crew and may overshadow other more critical alarms in the cockpit.
For this reason, at least one ground proximity warning system includes techniques that limit the number of potential nuisance alarms generated. This ground proximity warning system uses a look ahead distance value that defines a distance surrounding the aircraft in which the ground proximity warning system will provide alarms to the flight crew concerning potential ground proximity danger. This look ahead distance is further divided into advisory and warning regions, such that advisory alarms are generated for potential problems with terrain located at the outer extremities of the look ahead distance, while warning alarms are provided for problems with terrain located nearer the current position of the aircraft.
The look ahead distance value used by this ground proximity warning system is typically based on the flight parameters and flight characteristics of the aircraft. For example, the look ahead distance value may be based on the ground speed of the aircraft, actual roll angle of the aircraft, and/or a characteristic turning radius associated with the type of aircraft. The look ahead distance value may also be based on the proximity of the aircraft to a selected runway. Specifically, as an aircraft approaches a runway for landing, the look ahead distance value is typically decreased to reduce the number of nuisance alarms and focus on potential problems within the immediate surroundings of the aircraft.
In addition, the look ahead distance value is typically limited to minimum and maximum values. A minimum look ahead distance value is typically chosen to provide a minimum reaction time, while a maximum look ahead distance value is typically chosen to reduce the number of nuisance alarms generated.
As an example, one of the look ahead distance values typically used by the ground proximity warning system is based on the speed of the aircraft and a characteristic turning radius associated with the aircraft. This look ahead distance value is referred to as the ground speed look ahead distance value and typically provides a look ahead distance value based on the speed of the aircraft and the reaction time required for the aircraft to perform two turning radii plus an added reaction time. The equation for the determination of this ground speed look ahead distance is:
LADGround Speed=0.00278(Vg)+0.000050568(Vg2)+K
where
LAD=ground speed look ahead distance in nm,
Vg=ground speed in kts, and
K=constant.
The derivation of this equation is discussed in detail in U.S. Pat. No. 5,839,080 to Muller, entitled xe2x80x9cTerrain Awareness Systemxe2x80x9d which is assigned to the assignee of the present application. The derivation of this equation is also provided later below.
FIG. 1 depicts this equation graphically in terms of look ahead distance versus ground speed. As can be seen, for aircraft speeds in the range of 100 kts to 250 kts, the look ahead distance value varies with ground speed. For aircraft speeds below 100 kts, the ground proximity warning system limits the look ahead distance value to 0.75 nm. This lower limit is used to ensure that a proper minimum look ahead distance is maintained. The ground proximity warning system also limits the look ahead distance by an upper limit of 4 nm for speeds exceeding 250 kts. The upper limit of the look ahead distance value is provided to reduce the number of nuisance alarms generated.
Although limiting the distance surrounding the aircraft in which ground proximity warning alarms are generated is advantageous as it reduces nuisance alarms, there may be some drawbacks. Specifically, if the aircraft is traveling at a high rate of speed, of for example in excess of 250 knots, the limited look ahead distance may not provide desired reaction times for the aircraft to perform an evasive maneuver. For example, FIG. 2 illustrates a plot 2 of the reaction time as a function of aircraft speed and look ahead distance. Specifically, the plot depicts the reaction time provided by the look ahead distance for the given speed of the aircraft, i.e., Reaction Time=LAD/Speed.
As illustrated in FIG. 2, for aircraft speeds in the range of 100 kts to 250 kts, the reaction time increases, however, above 250 kts, where the look ahead distance is limited to 4 nm, the reaction time begins to decrease, shown by decreasing portion 4 of plot 2. For example, at 250 kts, the look ahead distance value provides a reaction time of approximately 55.5 seconds. However, at 360 kts, the reaction time has been reduced to 40 seconds. Thus, while the above approach provides a mechanism for reducing the number of nuisance alarms provided to the flight crew, with increased aircraft speeds, the reaction time may be reduced to a point where it is more desirable to provide at least some minimum reaction time.
One solution to the problem of reduced reaction time for increased aircraft speeds could be to increase the maximum limit of the look ahead distance value to a value greater than the current 4 nm maximum limit. This would allow for increased look ahead distance values at higher aircraft speeds. However, as illustrated in the above equation, the look ahead distance value is a function of the square of the speed of the aircraft. Thus, for small increases in aircraft speed, the look ahead distance value will increase significantly. As such, increasing the maximum look ahead distance limit above the current 4 nm maximum limit may undesirably increase the number of nuisance alarms.
In light of this, it would be desirable to have a ground proximity warning system that generates a look ahead distance that provides a desired minimum reaction time when the aircraft is traveling at higher speeds, while at the same time reducing the number of nuisance alarms generated.
As set forth below, the apparatus, methods, and computer program products of the present invention overcome many of the deficiencies identified with using a limited look ahead distance value for high speed flight. The present invention provides several apparatus, methods, and computer program products for determining a look ahead distance value for use in a ground proximity warning system. Specifically, the apparatus, methods, and computer program products of the present invention generate a candidate look ahead distance value that provides a constant reaction time for varying aircraft speeds. This constant reaction time look ahead distance value is compared to other candidate look ahead distance values that have variable reaction times for varying aircraft speeds. If the aircraft is traveling at a relatively high speed, the apparatus, methods, and computer program products of the present invention select the constant reaction time look ahead distance value for use in the ground proximity warning system.
Importantly, because the apparatus, methods, and computer program products of the present invention switch from a variable reaction time look ahead distance value to a constant reaction time look ahead distance value when the aircraft is travelling at higher speeds, the present invention can provide a look ahead distance value that reduces nuisance alarms for lower speeds, while also providing desired reaction times for higher speeds. Additionally, the constant reaction time look ahead distance value is selected such that it provides desired reaction times, while at the same time not necessarily enlarging the look ahead distance value to reduce the number of nuisance alarms at higher speeds.
These and other advantages are provided by an apparatus and method for computing a candidate look ahead distance value for use in a ground proximity warning system. The apparatus of this embodiment includes a processor that generates a constant reaction time look ahead distance value based on a selected constant reaction time and the speed of an aircraft. In operation, the processor receives a selected reaction time value representing a desired reaction time. The processor multiplies the selected reaction time value by the speed of the aircraft. This generates a constant reaction time look ahead distance value that provides the desired reaction time for evasive maneuvers at the current speed of the aircraft.
As discussed, the processor of the present invention multiplies the speed of the aircraft by a desired reaction time. In one embodiment, a reaction time of sixty seconds is desired for evasive maneuvers at high speeds. In this embodiment of the present invention, the processor generates the constant reaction time look ahead distance value by multiplying the speed of the aircraft by sixty seconds.
The present invention also provides apparatus and methods for determining, from a plurality of candidate look ahead distance values, a look ahead distance value for use in a ground proximity warning system. In this embodiment, the processor selects a look ahead distance value representing a distance extending about an aircraft within which the ground proximity warning system provides ground proximity alerts. Importantly, in this embodiment, one of the candidate look ahead distance values is a constant reaction time look ahead distance value that varies as the speed of the aircraft varies to provide a constant reaction time.
In one embodiment, at least one of the other candidate look ahead distance values is a variable reaction time look ahead distance value. The variable reaction time look ahead distance value varies as the ground speed of the aircraft varies and provides a variable reaction time. In this embodiment of the present invention, the processor selects one of the candidate look ahead distance values based on a mathematical relationship between the plurality of candidate look ahead distance values. For instance, in one embodiment, the processor selects the smaller of the candidate look ahead distance values as the look ahead distance value for use in the ground proximity warning system.
Importantly, in one embodiment, the constant reaction time look ahead distance value provides a desired reaction time for aircraft speeds that exceed a predetermined threshold speed. In this embodiment, if the aircraft speed is at least as great as the predetermined threshold speed, the processor selects the constant reaction time look ahead distance value as the look ahead distance value for use in the ground proximity warning system. Additionally, in one embodiment, if the selected look ahead distance value exceeds a predetermined maximum look ahead distance value, the processor limits the look ahead distance value to the value of the predetermined maximum look ahead distance value.
As discussed above, in one embodiment, at least one of the candidate look ahead distance values is a variable reaction time look ahead distance value. Depending on the embodiment, the variable reaction time look ahead distance value may be a distance to runway look ahead distance value, a ground speed look ahead distance value, and/or a roll angle look ahead distance value.
The present invention also provides computer program products for determining a look ahead distance value for use in a ground proximity warning system. The computer program products include a computer readable storage medium having computer readable program code means embodied in the medium. The computer-readable program code means includes first computer instruction means for selecting from a plurality of candidate look ahead distance values, a look ahead distance value representing a distance extending about an aircraft within which the ground proximity warning system provides ground proximity alerts. Importantly, one of the candidate look ahead distance values is a constant reaction time look ahead distance value that varies as the speed of the aircraft varies to provide a constant reaction time.
In one embodiment, at least one of the candidate look ahead distance values is a variable reaction time look ahead distance value. This variable reaction time look ahead distance value that varies as the ground speed of the aircraft varies and provides a variable reaction time. In this embodiment, the first computer instruction means comprises means for selecting one of the candidate look ahead distance values based on a mathematical relationship between the plurality of candidate look ahead distance values. For example, in one embodiment, the first computer instruction means comprises means for selecting the smaller of the candidate look ahead distance values as the look ahead distance value for use in the ground proximity warning system.
In a further embodiment, the computer-readable program code means further includes second computer instruction means for generating the constant reaction time look ahead distance value. In operation the second computer instruction means multiples the speed of the aircraft by a selected constant reaction time to generate the constant reaction time look ahead distance value.
In another embodiment, the computer readable program code means may further include third computer instruction means for comparing the selected look ahead distance value to a predetermined maximum look ahead distance value and fourth computer instruction means for limiting the look ahead distance value to the value of the predetermined maximum look ahead distance value, if the selected look ahead distance value exceeds the predetermined maximum look ahead distance value.